Capacitive sensors and probes are used in a great number of applications, for example to monitor a fluid level in a tank, to measure the distance between two moving parts of an object, to measure the vibration or wear of a rotating shaft, etc.
Known in the art are U.S. Pat. Nos. 4,261,397 (Guy)--4,311,959 (Riessland et al.)--4,347,741 (Geiger)--4,661,797 (Schmall)--4,675,670 (Lalonde et al.)--4,677,275 (Schmall)--4,682,272 (Furlong et al.)--4,766,368 (Cox)--4,912,662 (Butler et al.)--5,012,196 (Baranski)--5,014,011 (Colvin)--5,153,525 (Hoekman et al.)--5,166,679 (Vranish et al.)--5,235,217 (Kirton)--5,237,284 (Van Der Valk)--5,326,983 (Hejazi)--5,399,979 (Henderson et al.)--5,410,297 (Joseph et al.)--5,583,443 (McMurtry et al.)--5,610,528 (Neely et al.), showing examples of various types and configurations of capacitive sensors used in a multitude of applications.
Usually, in the case of a reading of a rotating shaft with a capacitive sensor, inductive technology involving EDDY currents is used. The drawback of this technique is that the user must calibrate or interpret the measurements of the sensor according to the type of material forming the shaft. Furthermore, a density variation of the material on the circumference of the shaft causes a variation of the measurement which is difficult to discriminate from the real circularity or vibrations to be measured. Other kind of applications involving the use of capacitive sensors are also subjected to the above drawback. Furthermore, the distance between the capacitive sensor and the reading/measuring apparatus usually causes some problems due to the impedance of the link between them.